This study uses a unique combination of experimental and satellite observations to document the characteristics of the relatively small regions of the atmosphere that are responsible for most of the meridional transport of water vapor from the tropics to the midlatitudes. These regions were recently referred to as "Atmospheric Rivers" due to their relatively small spatial scales and transport efficiency. An earlier study evaluated 3 years of output from the ECMWF global numerical model, and concluded that essentially all of the meridional water vapor flux occurred in less than 10% of the total longitudinal length at a given latitude (Zhu and Newell 1998, Mon. Wea. Rev.). These fluxes occurred primarily in the warm sector of extratropical cyclones, i.e., in the low-level jet (LLJ) where the highest water vapor content and strong poleward velocity are co-located.

Because of the importance of the LLJ in determining the location and intensity of rainfall in land-falling winter storms on the U.S. West Coast, the California Land-falling Jets experiment (CALJET) was carried out in the winter of 1997/98, during which unique data were collected concerning the LLJ. CALJET data collection and analysis have focused heavily on characterizing the LLJ and its impact on precipitation distributions.

This paper combines the unique airborne and wind profiler data collected in CALJET with satellite derived integrated water vapor retrievals. It uses an in-depth case study analysis documenting the horizontal and vertical distributions of the horizontal water vapor flux in a representative storm on 25-26 January 1998. This was accomplished using in situ and dropsonde data collected offshore by a NOAA P-3 research aircraft, which revealed that the moisture core and a majority of the horizontal water vapor flux occurred in a ~200 km wide swath. SSM/I polar-orbiting satellite retrievals of integrated precipitable water vapor were then examined for the CALJET winter of 1997/98. Based on an objective set of criteria, >100 cases were characterized as atmospheric rivers. These data were then analyzed to produce a statistical description of the width of the water vapor plume component of the atmospheric rivers. These case study and climatological results from observations are then compared with the conclusions drawn earlier from the evaluation of global model simulations noted above.